| With the fast development of national construction,the construction of transportation infrastructure such as expressway,railway and municipal engineering have become more and more extensive,and it is inevitable to contact karst areas with hidden caves.The existence of karst has a significant influence on the bearing capacity of the pile foundation.The karst caves destroy the integrity of the rock mass.The bearing mechanical behavior of the rock mass is different from the semi-infinite medium.The "stratum-pile foundation-karst cave" of the pile foundation in karst areas.The system’s failure mode,load transfer law,bearing characteristics,etc.are different from conventional rock-socketed piles.The stability of the pile foundation in karst areas will be greatly restricted by the development of karst.In this paper,relying on the Nanjing Third Ring Road Project,using on-site survey,theoretical analysis and numerical simulation methods,the stability of the underlying bead-shaped cavern bridge pile foundation are studied.The main research contents and achievements of this paper are as follows:(1)The distribution and development characteristics of karst in the project site are found out by means of field test method,high-density resistivity exploration,natural source micro motion exploration,geological drilling exploration and other exploration methods.The results show that the karst geological genesis of the site includes formation lithology,geological structure,hydrogeological conditions,climate and tunnel filling materials;The combination of high-density resistivity method and natural source micro motion method can find out the karst distribution more effectively;Beaded karst caves are widely distributed in the site,mainly double-layer karst caves.The height of karst caves is mainly 6m,the average thickness of roof is 6m,and the vertical spacing of karst caves is basically 6m.(2)Using the theoretical analysis method,the calculation formula of safe thickness of karst cave roof under different failure modes is reasonably deduced,the influence law of rock mass quality on safe thickness is studied,and the reasonable value range of safe thickness is given.The results show that the higher the quality grade of rock mass,the harder the roof rock mass,and the smaller the safe thickness value;Under the punching failure mode,for class I,II,III and IV rock masses,the roof needs to be greater than 0.94,1.80,2.67 and 3.92 times the pile diameter respectively;Under the shear failure mode,for class I,II,III and IV rock masses,when the pile diameter is greater than 1.0m,1.1m,1.2m and 1.3m respectively,the roof thickness of 3 times the pile diameter can meet the needs of the project;Under the bending tensile failure mode,for class I and II rock masses,when the pile foundation diameter is 0.8m～ 1.5m,the roof thickness of three times the pile diameter can meet the needs of the project.For class III and IV rock masses,when the pile diameter is less than 0.9m and 1.1m respectively,it is necessary to appropriately increase the thickness of karst cave roof according to the specification requirements to ensure the safety of the project.(3)Using the numerical simulation method,according to the situation of beaded karst cave,the overall model of "pile foundation rock stratum karst cave" is established,and the influence law of different parameters on the overall stability of "pile foundation rock stratum karst cave" is studied.The results show that the settlement mainly occurs in the pile foundation,the vertical stress is concentrated in the pile body and pile end,the soil strain mainly occurs in the rock layer above the karst cave,and the plastic zone of the rock layer mainly occurs in the upper karst cave roof,and gradually develops from the pile end and karst cave edge to penetration;When the load increases to 7000 k N,obvious punching failure body appears and gradually develops to penetration;When the quality grade of rock mass is reduced to grade III,obvious punching shear failure occurs,and the body gradually develops to penetration;The plastic zone develops gradually with the decrease of thickness span ratio.When the thickness span ratio is less than 0.6,there is an obvious punching failure body;The distance span ratio has little effect on the plastic zone of rock mass where the karst cave is located.(4)Combined with field engineering examples,the rationality of the method for determining the safe thickness of karst cave roof in this paper is verified.The results show that the provisions on the thickness of karst cave roof without considering the rock mass quality in the specification are slightly insufficient.When facing the weak rock mass with poor rock mass quality,the specified value of roof thickness of 3 times the pile diameter can not meet the requirements of roof stability;This method considers the different quality grades of rock mass,which can better target the weak rock mass with poor quality,and can better ensure the stability of roof in the value of safe thickness. |
